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XtOffsetOf()							  XtOffsetOf()

Name
  XtOffsetOf  -	 determine  the byte offset of a field within a structure
  type.

Synopsis
  Cardinal XtOffsetOf(structure_type, field_name)

Inputs
  structure_type
	    Specifies a type that is declared as a structure.

  field_name
	    Specifies the name of a field of the structure.

Returns
  The offset in bytes of the specified field from the  beginning  of  the
  specified structure.

Availability
  Release 4 and later.

Description
  XtOffsetOf()	is  a  macro  that  expands to a constant expression that
  gives the offset in bytes to the specified structure	member	from  the
  beginning of the structure.

Usage
  XtOffset()  is  usually  used	 to determine the location of an variable
  within a structure when initializing a resource list.	 It  is	 used  by
  widget  writers,  and	 anyone	 who needs to fetch application resources
  with XtGetApplicationResources().  Resource fields are defined in terms
  of offsets from a base address from the beginning of a widget.  Thus, a
  resource value can be kept up to date by the Resource	 Manager  without
  any  knowledge  of the instance structure of the widget; it uses just a
  relative byte offset.

  XtOffsetOf() is slightly more portable than XtOffset()  which	 performs
  the  same  function  but  takes  a pointer type rather than a structure
  type.

Example
  XtOffsetOf() is used to declare a resource list for  the  Athena  Label
  widget:

     #define offset(field) XtOffsetOf(LabelRec, field)
     static XtResource resources[] = {
	 {XtNforeground, XtCForeground, XtRPixel, sizeof(Pixel),
	     offset(label.foreground), XtRString, XtDefaultForeground},
	 {XtNfont,  XtCFont, XtRFontStruct, sizeof(XFontStruct *),
	     offset(label.font),XtRString, XtDefaultFont},
		 .
		 .

Background
  XtOffsetOf() is defined in terms of XtOffset() on many systems:

     #ifdef offsetof
     #define XtOffsetOf(s_type,field) offsetof(s_type,field)
     #else
     #define XtOffsetOf(s_type,field) XtOffset(s_type*,field)
     #endif

See Also
  XtOffset(1).

Xt - Utilities							  XtOffsetOf()

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